1. Field of the Invention
The present invention relates to an optical disc apparatus and, more particularly, to an optical disc apparatus reproducing information recorded on the optical disc.
2. Description of the Related Art
Generally, an optical disc apparatus is provided with a mirror circuit for detecting movement of an optical pickup between tracks formed on an optical disc. The mirror circuit is used for counting a number of tracks crossed by the optical pickup when a seek operation is performed so as to move the optical pickup to a target track.
FIG. 1 is a circuit diagram of a mirror circuit of a conventional optical disc apparatus. In the mirror circuit shown in FIG. 1, an RF signal reproduced from an optical disc is provided to an automatic gain control circuit (AGC) 24. A level of the RF signal is adjusted by the AGC circuit 24 as shown in FIG. 2-(A), and is provided to a bottom detecting unit 26.
The bottom detecting unit 26 holds minimum values of the RF signal shown in FIG. 2-(A), that is, the bottom detecting unit 26 performs a bottom-hold so as to detect a bottom signal shown in FIG. 2-(B). The bottom-hold is performed with a time constant so that the bottom-hold can be performed even when a traverse is performed at a high speed. The bottom signal is inverted by an inverting amplifier 28 with respect to a reference level signal so as to detect an amplitude change signal shown in FIG. 2-(C). The reference level signal is provided from a first fixed reference level signal source 22. The amplitude change signal is compared with another reference level signal provided by a second reference level signal source 32 by a comparator 30, the reference level signal having a level that is 60% of that of the amplitude change signal. Thereby, a mirror signal shown in FIG. 2-(D) is generated. The mirror signal is at a low level when an optical pickup aligns with a disc track, and is at a high level when the optical pickup is at a position between tracks or a defect is detected.
In the mirror circuit, a level of the bottom signal changes in association with changes in a reproducing speed such as a standard reproducing speed or a double reproducing speed. For example, when a reproducing speed is increased, amplitude of the RF signal is decreased.
As a result, a level of the amplitude change signal is decreased as shown in FIG. 2-(E). Accordingly, the reference level signal provided by the second reference level signal source 32 cannot corresponds to 60% of maximum amplitude of the amplitude change signal. Thus, a width of a pulse of the mirror signal is changed as shown in FIG. 2-(F). In the worst case, a pulse cannot be generated, which results in undetection of a traverse.
In a recordable optical disc such as a CD-R, dispersion may occur for each apparatus writing an optical disc. Accordingly, when information recorded on the optical disc is reproduced, dispersion may occur in the bottom signal in response to recorded portions written by different apparatuses. In such a case, the above-mentioned problem may also occur. Additionally, a dispersion may also occur in the level of the bottom signal when a CD-ROM (Compact Disc Read Only Memory) or a DVD (Digital Video Disc) is compatibly reproduced, which results in the above-mentioned problem.